The present invention relates to a method and apparatus for evaluating aberrations of an optical element such as optical head preferably for use with an optical playing/recording system.
Typically, a conventional method for evaluating aberrations of an optical element needs two steps; a first step for reconstructing an original wavefront from a shared pattern of two divided images and a second step for determining several aberrations individually from the reconstructed wavefront.
Specifically, FIG. 19 shows a conventional system generally indicated by reference numeral 300 for determining aberrations of an optical element 302. In determining aberrations of an optical element, light from the optical element 302 is transmitted into a first beam splitter 304 where it is divided into first and second lights 306 and 308. The first light 306 passed through the first beam splitter 304 is reflected at a first mirror 310 and then transmitted through a second beam splitter 312 into an image receiver 314. The second light 308 reflected by the first beam splitter 304 is reflected by a second mirror 316 and the second beam splitter 312 into the image receiver 314. The second mirror 316 is positioned so that the first and second lights 306 and 308 are shifted from the other on the image receiver 314 to form a sharing image or pattern thereon. The sharing image or pattern is then analyzed at an image processor 316 to determine aberrations of the optical element.
With this arrangement, the original wavefront is determined from the sharing image or pattern, which requires many steps for evaluating the aberrations and therefore is time consuming. Also needed is an analysis of second order matrix, which requires a great number of calculations. Likewise, a calculation for determining the original wavefront from the sharing image requires a great number of steps and therefore is also time consuming. Further, the light is divided into two and then the divided two light must be overlapped on the image receiver 314 with a great precision, which requires the respective light paths to be held positively and therefore renders the arrangement so bulky.
FIG. 20 shows another conventional system generally indicated by reference numeral 318 for evaluating aberrations of an optical element 320 to be adjusted. With the system 318, light 322 is transmitted through an objective lens 324 of the optical element 320 to a transparent plate 326. The light 322 is then focused by a collecting lens 328 as a light spot on an image receiver 330 which forms a series of signals corresponding to the received image. The signals are then transmitted to a signal processor 332 where a distribution of light intensity in the received image is determined. The distribution of the light intensity is used for determining aberrations of the optical element 320, and the determined aberrations are in turn used for adjusting the optical element 320.
In this instance, however, the focused light spot should be greatly magnified and therefore a field of view of the image receiver 328 is so narrow. This means that even a small translation of the image spot would cause the light spot to move out of the field of view of the image receiver, which fails to detect the aberrations. Also, the spot light includes no phase information, which makes it difficult to obtain aberrations precisely.
Therefore, an object of the present invention is to provide an improved method and apparatus capable of determining aberrations of an optical element easily.
For the purpose, according to a method for evaluating an aberration of an optical element, light is transmitted through the optical element and then diffracted into 0, xc2x11, xc2x12, . . . order diffraction lights, for example. Among others, first and second lights (e.g., 0 and +1, 0 and xe2x88x921, +1 and xe2x88x921, or 0 and xc2x11 order diffracted lights) are overlapped to form an image shared by the first and second lights. Then, light intensity at first and second points in the shared image are detected. At this moment, light intensity at the first and second points are changed. Then, a phase difference in light intensity of between first and second points is determined. Using the phase difference, aberrations of the optical element are determined.
In another aspect of the present invention, a plurality of points are determined in the shared region. Specifically, determined are first to seventh points. The first is a mid-center of a first line connecting axes of the first and second diffracted lights. The second point is located on a second line crossing the first line at the first point. The third point is located on the second line so that second and third points are positioned symmetrically with respect to the first line. The fourth and fifth points are located on the second line and symmetrically on opposite sides of the first line so that each of fourth and fifth points is spaced a distance from the first line. Sixth and seventh points are located on opposite sides of the first line so that each of sixth and seventh points is spaced the distance from the first line.
In another aspect of the present invention, the method includes steps for determining comma of the optical element. To this end, a first phase difference Ph(1) in light intensity of between first and second points is determined. Likewise, a second phase difference Ph(2) in light intensity of between second and third points, a third phase difference Ph(3) in light intensity of between fourth and fifth points, a fourth phase difference Ph(4) in light intensity of between sixth and seventh points are determined. Using such phase differences, a magnitude of comma is determined by a phase difference obtained from the following equation:
Phase difference=|Ph(1)|xe2x88x92|Ph(2)|/2
Also, a direction of comma is determined using a phase difference obtained by the following equation:
Phase difference=|Ph(4)|xe2x88x92|Ph(3)|
In another aspect of the present invention, astigmatism of the optical element is determined. In this determination, a diffraction grating is directed in three directions. For each direction, light is transmitted through the optical element and then guided into a diffraction grating to obtain first and second diffracted lights. The first and second diffracted lights are overlapped each other to form a shared image. Then, an intensity of light are determined at first and second points in the shared image. The first and second points are located on a line crossing a mid-center of another line connecting centers of the first and second diffracted lights and symmetrically with respect to another line. At this moment, light intensity is changed. Further, a phase difference in light intensity of between first and second points is determined, which is used for evaluating an astigmatism of the optical element.
An apparatus for evaluating an, aberration an optical element has a reflection type or transmission type of diffraction grating. The grating is formed with a is plurality of parallel grooves so that light from the optical element is diffracted into diffraction lights. The diffraction lights include first and second lights partially overlapped to form a shared image. A mechanism is provided for moving the diffraction grating in a direction substantially perpendicular to an axis of the light. The sharing image is then received by an image receiver. A phase of light intensity at each of plural points in the shared image is determined and then used for evaluating aberrations.
Another apparatus for evaluating an aberration an optical element includes a pair of first and second transmission type of diffraction gratings. Each of first and second gratings is formed with parallel slits to diffract light into diffraction lights rather than zero order diffraction light. The first and second diffraction gratings are positioned parallel to each other with the slits directed in one direction to form a shared image in which two diffracted images are partially overlapped. A mechanism is provided for moving the first diffraction grating in another direction that forms a certain angle with the one direction. Also, provided are an image receiver for receiving the shared image and a processor for determining a phase of light intensity at each of plural points in the shared image.
In addition, an apparatus for correcting an aberration an optical element includes a mechanism for correcting aberration of the optical element.